The Climate, Life, and Landscape of Jurassic Time

The geologists who attempt to reconstruct the geography and climate of the Jurassic Period first gather all possible facts and try to fit them together to form a logical pattern. The results are then examined for weak points and an attempt is made to find field evidence throwing light on these weak points. The final result represents the sum of our knowledge at the time but is subject to change as new facts are obtained. Thus the following outline represents present thinking that may be changed somewhat by future studies.

The land for miles around the Dinosaur Quarry was a low-lying desert in early Jurassic time. The mountains you see now had not yet been formed, and the whole desert area lay close to sea level. Great restless sand dunes drifted across this level land to form a blanket 700 feet thick. As the earth's crust sank, these dunes were covered by a long arm of an arctic sea that extended southward along the present trend of the Rocky Mountains across Canada, Montana, Wyoming, and Utah. Millions of years later, in late Jurassic time, when the Morrison formation was deposited, the area rose again and the stage was set for the dinosaurs.

Imagine if you can, the vast plains extending from Mexico to Canada and from central Utah to the Mississippi River. To the west were high mountains in the Great Basin region of Nevada and western Utah. From these highlands flowed great sluggish streams that carried large amounts of sand and silt. Since the plains were almost flat, swamps and small lakes were probably numerous. The streams may have changed their courses from time to time as they were not confined to deep valleys. When the whole region emerged from the sea the climate became more humid. Volcanoes were active for to the west; the winds carried clouds of ejected dust eastward and deposited them on the plains. Semi-tropical conditions probably existed throughout the United States and in parts of Canada. These deposits are called the Morrison formation.

The warm humid climate provided ideal conditions for plant growth. Great forests of lush vegetation covered the land. Many of these plants have since disappeared, but some of their related species may be found today in the tropics. Most of the plants of our Temperate Zone had not yet evolved. However, there were tall stands of a type of pine, and other evergreens. There were also gingkos and curious tree ferns.

Various herb ferns formed a ground cover as thick and lush as grass on a well-watered prairie. Palmlike ferns resembling today's cycads were common, while along the river grew horsetail rushes like those living today. Flowering plants of the Recent Epoch of geologic time (in which we are now living) had not yet made their appearance. Thus the hardwood, broadleaf forests of oak, elm, beech, maple, and similar trees were absent. So too were the flowering shrubs familiar to the Temperate Zone. Even the grasses were missing.

However, if you could picnic in this strange plant world you would soon be slapping mosquitos and cursing the ants. For even in such ancient times these insects were present; and so were a great variety of other insects as is known from the more than 1,000 species that have been discovered in Jurassic rocks. Among them were representatives of most modern orders such as grasshoppers, beetles, moths, ants, and flies. Jurassic insects probably looked much like those of today.

Among the most interesting of the strange reptiles were the pterosaurs that dominated the skies. They resembled the modern bats in some ways but their leathery wings were supported on each side by one finger instead of four, and their skins were scaly or bare instead of hairy. Some forms had long tails that were flattened at the tip and helped them balance in flight, but others were tailless. Some pterosaurs were no larger than sparrows while others had wing spans of 3 to 4 feet.

Crocodiles sunned themselves on the banks of sluggish streams and lakes. They probably looked a good deal like those that live in modern swamps and their habits were similar. Many a small dinosaur fell victim to their stealthy attack and disappeared beneath the waters of some ancient stream.

Birds have been found in Upper Jurassic rocks of Germany and may have lived here too. Their fossil remains would probably have been classed as reptiles had not feather imprints been a part of them. About the size of crows, these reptile-like birds had small conical teeth, three-clawed fingers on each wing, and a long tail instead of the fan of feathers seen on modern birds.

Small mammals were also living at the time the Morrison formation was being deposited and their remains have been found in the dinosaur quarry at Como Bluff, Wyo. The largest ones were about the size of a house cat but the majority were much smaller, probably about the size of today's mice.

We do not know much about the habits of these early mammals but they were probably rather shy and retiring. This would be expected in the world of giants where they lived. Some of them lived in trees and there was one group whose skull characteristics resemble those of the rodents. It is likely that these primitive mammals lived a life similar to that of the rodents millions of years later.

This, then, was the setting, the stage upon which the dinosaurs played their leading roles. Although we have restricted our discussion to Morrison time in northeastern Utah, the same or similar animals lived all over the world. Worldwide humid and mild climates produced a similarity of plant and animal life during most of the Mesozoic Era whose like has not been seen in the last 60 million years. It was a strange world and ruled by strange animals, but it must have been an interesting one.

HOW DO WE KNOW?

This is a good time to stop and try to explain that this story, of plants and animals of the past, has a firm foundation in today's facts—it is nor a fantasy.

The methods by which geologists and paleontologists have established the age, climate, and life of Morrison time cannot be described for you here in detail. To attempt such a description would require too much space and would probably seem dull to most readers. Perhaps the best approach is to describe some features and explain how they contribute to our knowledge.

The rocks that were deposited here in Morrison time tell us much of the story. The sandstones were once stream sandbars or perhaps beaches around lakes. The shale, siltstone, and clay were muddy stream or lake bottoms. The discontinuous ledges of conglomerate probably represent gravel bars formed during flood stages or at places where the stream currents were very swift.

Just rocks you may say—but look closely. A piece of sandstone contains grains of sand that differ from each other in size, shape, and composition. Frequently these characteristics point to the source of the sandstone and tell something about the conditions at the time it was deposited. Chunks of black material are examined closely and prove to be charcoal—carbonized remains of plants.

Microscopic examination of clay fragments reveals shards of volcanic glass and ash that speak of active volcanoes. Sometimes these clays bear the carbonized imprints of delicate plants that long ago sank to the bottom of some lake or stream where they were buried and fossilized.

The fossils themselves are most important in reconstructing conditions of the past. We find the shells of fresh-water clams in the sandstones with dinosaur bones. Crocodile bones are also common. We are reasonably sure, then, that these deposits of sand and mud were formed in rivers and lakes when the climate was mild.

We reason by analogy. For example, fossil plants and animals have counterparts or descendants in the world of today. We assume, in the absence of contrary evidence, that the fossil animals lived like their present-day counterparts. Although no birds, mammals, or pterosaurs have been found in this quarry, they were probably living here with the dinosaurs. It is possible, in fact probable, that some modern animals and plants live in different environments than did their Morrison ancestors and relatives but we have no way of knowing which ones they were. We can only take the data available, arrange them as logically as possible, and continue the search for more. Some may scoff at such methods of reasoning yet they do provide good results. What other methods can be used when the world under investigation lies millions of years in the past?